Quality control method for inertial welding

ABSTRACT

A method and apparatus for maintaining quality control in a friction welding operation where two workpieces are relatively rotated while being constantly pressed into engagement at a common interface. One or more operating conditions, including axial upset at the common interface, are monitored and a signal is substantially instantaneously produced to indicate correlation between the monitored operating conditions and respective ranges for those conditions which are predetermined as being representative of an effective bond between the workpieces. In a first embodiment, operating conditions of axial upset, pressure of engagement between the workpieces and relative rotating speed are simultaneously monitored. In a second embodiment, axial upset is monitored in selectively delayed relation to commencement of the weld in order to provide improved quality control over inertia welding.

This is a continuation of Ser. No. 513,523, filed Oct. 10, 1974, nowabandoned which is division of Ser. No. 474,844 filed May 30, 1974 nowU.S. Pat. No. 3,888,405; which is a continuation-in-part of U.S. Pat.application, Ser. No. 286,312, filed Sept. 5, 1972 and now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for maintainingquality control in a friction welding operation, particularly inertialwelding operations for which a simplified version of the presentinvention is especially adapted.

The invention also relates, possibly in a more specific way, tocomputerized apparatus which substantially instantaneously provides asignal indicating whether one or more operating conditions are withinpredetermined limits indicating an acceptable bond formed at theinterface of the workpieces. In association with a friction weldingmachine including control means for automatically repeating a frictionwelding operation, the computerized apparatus may be employed toterminate operation of the machine if an operating condition is outsideof acceptable limits during a welding operation.

A brief review of different types of friction welding is set forth belowin order to more accurately define the present invention. Two types offriction welding are commonly referred to as conventional frictionwelding and inertial welding.

In a conventional friction welding operation, one workpiece is rotatedagainst another workpiece at a relatively constant speed and under arelatively constant axial load until the interface between theworkpieces is heated to a plastic condition. Relative rotation betweenthe workpieces is then rapidly stopped. The axial load may then beincreased to improve the strength of the bond at the interface.

Inertial welding refers to a process wherein energy for accomplishing abond between two workpieces is provided by a rotating inertial mass. Theinertial mass is accelerated to a selected speed to store apredetermined amount of energy. The workpieces are then pressed togetherby an axial load while inertial energy stored in the rotating mass isconsumed at the interface to produce frictional heating and plasticworking of the interface. The rotational speed of the inertial masscontinuously decreases and the entire energy of the inertial mass ispreferably consumed at the interface of the workpieces. However, incertain applications, only a selected portion of the energy in theinertial mass is consumed at the interface.

The present invention is particularly concerned with the specificoperation described above. For example, the type of bond accomplished byinertial welding may be simulated as illustrated in one instance by U.S.Pat. No. 3,542,274, issued on Nov. 24, 1972 and assigned to the assigneeof the present invention. That patent refers to a speed-programmedfriction welder wherein the relative speed of rotation between twoworkpieces is closely controlled by electronic means.

A comparison of the speed-programmed weld cycle of the above-notedpatent with an inertial welding operation indicates that both operationsare generally characterized by the relative speed of rotation betweenthe workpieces being varied in a substantially predetermined manner froman initial, relatively high speed with low torque at the interface to arelatively low speed with high torque at the interface, a bond beingsubstantially completed between the workpieces at their interface asrelative rotation ceases.

SUMMARY OF THE INVENTION

The present invention particularly contemplates a method and apparatusof quality control in an inertial welding process or similar weldingoperation characterized by two workpieces being relatively rotated inrubbing contact at a common interface while being continuously pressedtogether, the relative speed of rotation between the workpieces varyingin a substantially predetermined manner from an initial, relatively highspeed with low torque at the interface to a relatively low speed withhigh torque at the interface, a bond being substantially completedbetween the workpieces at their interface as relative rotation ceases.Within such an operation, metallurgical quality of the bond has beenfound to be closely related to the amount of upset or relative axialmovement occuring between the workpieces at their interface during thewelding process.

Accordingly, to provide a substantially instantaneous indication of weldquality, the present invention contemplates monitoring the amount ofrelative axial movement between the workpieces during the weldingprocess after the workpieces are initially engaged and providing asubstantially instantaneous signal indicating correlation between theactual upset and an upset range predetermined as being indicative of aneffective bond between the workpieces.

Preferably, the amount of upset is measured following a suitable delayafter initiation of the weld cycle, the amount of delay beingselectively variable depending on the characteristics of the workpiecesand the particular parameters of the weld cycle, in order to providemore accurate quality control in the finished weld.

The invention more particularly contemplates a computerized controlapparatus for maintaining quality control in friction welding machinesof either the inertial or conventional type including transducer meansfor monitoring the conditions of relative rotating speeds between theworkpieces, fluid operating pressure urging the workpieces intoengagement and axial upset at the interface of the two workpieces duringthe weld cycle, computer means receiving signals from the respectivetransducer means with signal means being coupled to the computer meansfor providing a substantially instantaneous signal corresponding to eachof the monitored conditions.

Other objects and advantages of the present invention are made apparentin the following description having reference to the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of an inertial welding machine andquality control apparatus according to the present invention, thequality control apparatus also being illustrative of a method accordingto the present invention.

FIG. 2 is a fragmentary view of a preferred signal means within suchquality control apparatus.

FIG. 3 is a schematic representation of an alternate embodiment ofquality control apparatus according to the present invention, theapparatus of FIG. 3 being illustrative of an alternate method ofoperation according to the present invention.

FIGS. 4A, 4B, and 4C sequentially illustrate the axial displacementbetween a pair of workpieces during a typical inertial welding cycle.

FIG. 5 graphically represents both the actual sequential axialdisplacement which is pictorially illustrated in FIGS. 4A-4C as well asan electronic signal representative of upset displacement between theworkpieces.

DESCRIPTION OF THE EMBODIMENT OF FIG. 1

The method and apparatus of the present invention is described withparticular reference to an inertial welding machine of the typeindicated at 11 in FIG. 1. The welding machine 11 has a frame 12supporting a rotatable spindle 13 and a movable tailstock assembly 14.One workpiece WP1 is supported for rotation by a chuck 16 mounted uponthe spindle 13. The outer workpiece WP2 is supported by a non-rotatingchuck 17 which is secured to the tailstock fixture 14.

A replaceable flywheel 18 is also mounted upon the spindle 13 forrotation with the one workpiece WP1 to provide a variable inertial massselected in accordance with conventional parameters which are well knownto those familiar with inertial welding technology. The spindle 13 isdriven in rotation by a hydraulic motor 19 with hydraulic fluid underpressure being supplied to operate the motor 19 by a hydraulic powersource 21.

Axial motion of the tailstock fixture 14 upon the frame 12 is controlledby a hydraulic ram 22 including a cylinder 23 secured to the frame 12. Arod 24 is secured to a movable piston 26 in the cylinder and penetratesone end of the cylinder 23 for connection to a bracket 27 which isaffixed to the tailstock assembly 14.

The machine described above is illustrative of friction weldingoperations where two workpieces are relatively rotated while being urgedinto engagement with each other to form a bond at their interface. Ininertial welding operations, energy for accomplishing the bond ispreferably stored in the rotating mass 18 by operation of the motor 19.The workpieces WP1 and WP2 are then brought into axial engagement byretraction of the piston 26 within the cylinder 23 to shift thetailstock assembly 14 leftwardly towards the spindle. Energy from theflywheel 18 is consumed at the interface of the workpieces, the relativespeed of rotation between the workpieces varying in a substantiallypredetermined manner from an initial, relatively high speed with lowtorque at the interface to a relatively low speed with high torque atthe interface, a bond being substantially completed between theworkpieces at their interface as relative rotation of the workpiecesceases.

The bonded workpieces may be left in the chucks 16 and 17 for arelatively short period of time, referred to as "cooling dwell" anddenoting a period of time necessary to adequately cool the bondedworkpieces in order to preserve dimensional stability after they areremoved from the machine.

Operation of the hydraulic source 21 and the hydraulic ram 22 arepreferably regulated by an automatic control assembly indicated at 28with an electrical power source 29 and including manually operatedcomponents or switches (not shown) to initiate and terminate variouswelding operations. Preferably, the machine controls are of the type forautomatically repeating operation of a welding operation within themachine 11. A control mechanism of somewhat similar character andillustrative of the control assembly 28 is described in U.S. Pat. No.3,542,274.

As noted above, during a friction welding process for joining metalworkpieces, certain parameters or conditions of the welder operation arecritical to the formation of a proper bond between the workpieces. Theseconditions are variable depending for example, upon the cross-section ofthe workpieces and their composition. However, the conditions may bepredetermined for any particular welding operation. Conditions which arecritical parameters in friction welding operation include the relativespeed of rotation between the two weld pieces during the weldingoperation. In the machine illustrated in FIG. 1, the relative speed ofrotation between the weld pieces is provided by the operating speed bythe spindle 13. Another condition is the pressure with which theworkpieces are axially urged together during the welding operation. Thispressure corresponds substantially to hydraulic pressure in thehydraulic ram 22.

Another condition is the amount of axial upset occuring at the interfacebetween the workpieces WP1 and WP2 which may be determined by the amountof relative axial movement occuring between the workpieces during thewelding process after the workpieces are initially engaged. Thiscondition is of particular importance as an indication of the quality ofa bond formed between the workpieces. Further, the amount of upsetoccuring during the welding process is particularly indicative of thequality of a bond performed by an inertial welding operation in amachine such as that illustrated in FIG. 1.

The present invention contemplates the use of transducers such as thoseindicated generally at 31, 32 and 33 for respectively monitoring axialupset between the workpieces, relative rotating speed of the workpiecesand pressure with which the workpieces are urged toward each other.Monitoring signals from these transducers are conditioned by separateanalog-to-digital converter means 34, 36 and 37 as well as a computerinput logic interface 38 for transmission to a digital computer 39. Forexample, a signal corresponding to each of the monitored conditions maybe instantaneously displayed for example, by means of an oscilloscopeindicated at 41. A permanent record of the operating conditions may alsobe provided by permanent recorder means 42 which may be a digital taperecorder, typewriter or digital recorder, for example.

Still further, a computer output logic interface 43 may operate aplurality of lamps 44, 46, and 47 for example, to provide aninstantaneous indication of quality within a bond completed by themachine 11. Additionally, the output interface means 43 may also beemployed to signal the control assembly 28 to interrupt operation of theweld machine 11 when operating parameters indicate an unsatisfactoryweld.

To review the components discussed above in greater detail, thetransducer 31 may be a linear displacement or position transducer whichis mounted upon the frame 12 by a bracket 51 and has a movable member 52secured to the tailstock fixture 14. The transducer 31 is preferably alinear variable displacement transducer (LVOT) coupled with theconverter 34 to provide a DC voltage signal proportional to axialmovement between the workpieces 16 and 17 after they are initiallyengaged by the hydraulic ram 22. The DC signal from the transducer 31 isdigitized by the converter 34 and delivered to the computer 39 throughthe input interface 38.

Similarly, the transducer 32 may be a tachometer for providing a DCvoltage proportional to rotating speed of the spindle 13. The DC signalfrom the transducer 32 passes through converter means 36 which performsa shaping and counting function with the resulting signals being passedto the computer 39 through the input interface 38. The transducer 33provides a DC voltage signal proportional to pressure in the hydraulicmeans 32 with which the workpieces WP1 and WP2 are axially urgedtogether during the welding operation. The signal from the transducer233 is digitized by the converter 37 and passed to the computer 39 bymeans of the input interface 38.

Additional synchronizing information is also delivered to the computer39 by the branched connection from the control assembly 28 as indicatedat 53. This synchronizing information is designed to condition thequality control apparatus so that various components in the qualitycontrol apparatus are properly adjusted to monitor the selectedconditions. The synchronizing information includes the time at which theweld is commenced, the time at which upset pressure is developed betweenthe workpieces WP1 and WP2 after their initial engagement, thecommencement of the cooling dwell period at which time it is no longernecessary to monitor the various conditions and the time at which theweld is completed. This final synchronizing signal may be employed toreset or adjust the entire quality control combination for a new weldingoperation.

The digital computer 39 always serves the functions of permanentlystoring "nominal" weld information, processing and storing productionpart information during each type weld operation and providing a signalthat indicates whether the upset length is too short, nominal, or toolong. Although primary machine variables of the type discussed abovewill normally be displayed, it may be desirable to display computedvariable (e.g. torque) which can be calculated from the input data andwelder physical constants. When collecting nominal part information fora particular type welding operation, the digital computer 39 is causedto hold the machine information permanently at the end of each cycle.During production operation of the display, all data pertaining to thecurrent type operation is destroyed as the collection of data for thenext type operation commences.

The computer 39 and various converters employed in the quality controlcombination may be of generally well known construction to perform thefunctions described above. As noted above, an output signal from thecomputer 39 may be fed to the instantaneous display means 41, thepermanent recording means 42 or the output interface means 43.

The present invention particularly contemplates the provision of asignal indicating correlation between the monitored conditions andranges for each of those conditions which are predetermined as beingrepresentative of an effective bond between the workpieces. One methodof providing such a signal is illustrated for example in FIG. 2. In thatfigure, the graticule or shield 61 in the oscilloscope has dashed linesas indicated generally at 62, 63 and 64 which indicate an acceptablerange for trace signals corresponding to the various conditions ofrelative speed, axial upset and pressure. With the horizontal sweep rateand vertical gain adjustment of the oscilloscope being properly set fora particular welding operation, the trace for each of the monitoredconditions will fall within the corresponding range during a properlycontrolled welding operation with the operator being able to instantlyobserve any deviation of the monitored condition from suitable limits.It is noted that other means could perform a similar function. Forexample, the computer means 39 could be programmed to compare each ofthe traces with predetermined limits in order to detect any deviation inthe monitored conditions.

The lamps 44, 46, and 47 which are coupled to the computer means 39through the output interface 43 provide an additional means ofindicating an abnormal condition during a welding operation. Forexample, the lamps in FIG. 1 are respectively actuated to determinewhether upset between the workpieces as monitored by the transducer 31is within a satisfactory range, exceeds the acceptable predeterminedrange or falls below the predetermined range. Similar sets of signallamps could also be employed for the other monitored conditions such asoperating speed and upset pressure.

It is also contemplated that the instantaneous display means 41 is anoscilloscope of the storage type so that the various traces indicatingeach of the monitored conditions remains upon the scope until a manualreset button (not shown) is actuated to condition the oscilloscope 41for a new weld cycle. Thus, the oscilloscope 41 provides the operatorwith a means of studying the character of traces for any given weldingoperation.

During operation of the circuit illustrated in FIG. 1, the inputinterface 38 and computer 39 are responsive to the simultaneous presenceof "start weld"(machine reaches weld speed) and "upset pressure" (partstouch) signals from the control assembly 28 to store informationrelating to "upset", "speed" and "pressure". At the same time, signalsfrom the respective transducers indicated at 34, 36, 37 are transmittedto the visual display 41 during the weld period. The visual display 41as also indicated above, may be a storage oscilloscope with markedgraticule to define parameter limits.

The computer circuitry also retains the upset signal from 34, that ispresent when the parts touch, as a reference. When the weld is completeand the "cooling dwell" signal is received by the input interface 38,the computer circuitry compares the upset signal present at that timewith the previously stored upset reference and holds the differencesignal (total upset) in memory. Information relating to speed andpressure may also be stored at that time.

The "weld complete" signal conditions the computer circuitry to releaseall stored information to the recorder device 42 and simultaneouslyfeeds the total upset signal to the output logic interface 43 whichindicates whether total upset is in the acceptable range. Acceptableupset is visually indicated by one of the lamps 44, 46, or 47 which alsoserve to indicate if actual upset is above or below an acceptable range.At the same time, the output interface 43 will produce a shutdown signalwhich conditions the machine controls 28 to prevent automaticallyrecycle of the weld machine 11.

The Embodiment of FIG. 3

The computer circuitry, recorder 42, display 41 and output interface 43function in substantially the same manner described above with referenceto FIG. 1. The computer circuit is conditioned by the same signalsexcept that when weld speed is reached, the "start weld" signal istransmitted through a time delay circuit or timer unit 70 arrangedbetween the transducer 34 and input interface 38. Thus, although thecomputer is ready to receive and process data when the "upset pressure"signal is present actual monitoring is not commenced until after apredetermined delay established by the timer 70 at which time the upsetsignal is passed to interface 38.

The timer 70 selectively delays monitoring of the upset signal to allowadequate time for the hydraulics to cause engagement between theworkpieces and for rough burn-off to occur at their interface before asignal is passed to the computer circuitry to establish the upsetreference. Therefore, the upset measurement signal seen by devices 41,42, 43 represents that part of the change in output voltage of the LVDT31 that occurs after the parts engage and the interface between WP1 andWP2 is properly conditioned for the start of upset measurement.

The time delay circuit 70 could be triggered for example, by the "upsetpressure" signal but a relatively short delay period would then benecessary. Accordingly, the timer 70 is preferably triggered by the"start weld" signal since a longer and more accurately determined timedelay is then involved.

The timing function could be achieved by any of several commercialelectrical, electronic, electro-mechanical or electro-pneumatic devicescapable of being triggered by an electrical signal. However, the timer70 is preferably of an electronic type employing an adjustable RCnetwork as an input to actuate a unijunction transistor, for example,because of its excellent accuracy and response. Such an electronic timeris manufactured by Eagle Signal Co., as Model CG2A602.

The relative positions of the workpieces in FIGS. 4A 4B and 4C areillustrative of upset occurring during a weld. The abutting surface ofworkpiece WP1 is rough and uneven while that of WP2 is relatively smoothand well defined. The dimension B indicates that approximately 1/8 inchof movement WP2 must occur to burn off surface projections on theworkpiece WP1 before metal-to-metal contact is assured over the totalinterface between the two workpieces.

FIG. 4B represents the condition of the workpieces where it is proper tostart upset measurment. WP2 has moved leftwardly a distance B' that isslightly greater than B which causes a slight amount of flash andestablishes a condition where there is even heating over the entireinterface area.

FIG. 4C shows the completed weld with approximately 1/4 inch ofmonitored upset or displacement. During this operation equal amounts offlash are produced from both workpieces so that each is foreshortened byapproximately 1/8 inch.

FIG. 5 graphically represents the same weld indicated in FIGS. 4A-4Cwith fixed representative parameters as noted below and wherein the timedelay is adjustable. The LVDT 31 is capable of measuring 1 inch ofdisplacement and its output changes 80 millivolts for each 0.1 inch ofdisplacement. The pre-bond gap between weld pieces is 0.25 inch, theweld cycle from application of ram pressure to completion of weld is 0.5sec., the hydraulic system causes the parts to engae in 0.3 seconds.Desired burn-off distance is 0.1875 inches and desired upset is 0.3inches. Time delay is set at 0.4 seconds.

Curve A shows the change in voltage output of the LVDT 31 from the timeaxial movement of workpiece WP2 commences until the end of the weldcycle. Curve B represents the amount of upset that would be measuredwith the time delay being varied from Zero time (ram pressure applied)to 0.5 seconds (end of weld cycle).

For example, if the timer were set at Zero delay, measurement wouldstart with pressure application to initiate movement of the workpieceWP2 and the measured upset would include initial separation between theworkpieces, burn-off and upset or a total of approximately 0.7375inches. With the time delay set at 0.3 seconds, measurement would startat approximately the time the pieces touch and the measurement would be0.4875 (burn-off plus upset).

When the parts engage, it may be seen from Curve A that tailstock motionslows and Δ LVDT Output is negligible during the initial part of theburn-off period (trace portion A₁). Subsequently, some upset occurs tillthe parts are in total contact at the interface (portion A₂).Thereafter, the rate of tailstock motion is reduced and Δ LVDT Output isagain negligible for a short period (portion A₃) while the parts heat.The Δ LVDT Output then rises sharply in the ensuing upset period(portion A4).

With the time delay set to 0.4 seconds, the upset reference isestablished as the instant, output level of the LVDT at that time. Thissignal value is compared with the voltage level at the end of the weldwhen the LVDT output is no longer changing to give total upset.

In most instances, an upset tolerance of ±.005 to ±.010 inches isacceptable in order to maintain relatively high quality control over theweld. It will be noted that the line B representing theoretical upset isa mirror image of the line representing Δ LVDT output so that lineportion B1 corresponds to line portion A3. It may therefor be seen thatthe time delay could be set at any point between approximately 0.37 and0.4 seconds and the final upset measurment would still remain withinacceptable limits for the particular parameters of the illustrated weldcycle.

It is desirable to set the time delay at a point to insure that all ofthe final upset is measured.

For any particular welding operation, the time delay may be determinedby running 5 or 6 weld cycles and comparing upset derived frommeasurement of the joined workpieces with the monitor reading. Apredetermined delay setting may then be maintained for the particularoperation. Generally, a time delay as short as 0.15 seconds is properfor parts of small cross section such as valve stems or thin wall tubingwhile the delay may be as long as one second or more for applicationsinvolving parts of greater section such as 2-4 inch diameter solid rodor large diameter, heavy walled tubing.

We claim:
 1. In an inertial welding process of the type wherein, duringeach of the plurality of repetitive cycles, two workpieces arerelatively rotated in rubbing contact at a common interface solely bymeans of a rotating inertial mass while being continuously pressedtogether, an improved method of maintaining quality control duringrepetitious cycles of said inertial welding process comprising the stepsof:Monitoring the amount of relative axial movement between theworkpieces after initial engagement of the workpieces to determine axialupset during the welding process; Comparing axial upset amount with anupset range predetermined as being representative of an effective bondbetween the workpieces. Producing a substantially instantaneous signalindicating correlation between the axial upset amount and thepredetermined upset range; Monitoring the relative speed of rotationbetween the two workpieces and pressure under which the two workpiecesare axially engaged, simultaneously with monitoring of axial movementbetween the parts; Comparing the relative speed of rotation and theaxial pressure with respect to predetermined ranges which arerepresentative of an effective bond between the work pieces; Productingsubstantially instantaneous signals indicating correlation of themonitored relative speed of rotation and axial pressure with thepredetermined ranges for these conditions; Conditioning repetition ofthe welding operation upon maintenance of axial upset amount, relativespeed and axial engagement pressure within their respectivepredetermined ranges during each cycle; Permanently recording axialupset amount, between the workpieces; and Displaying a continuous signalfor each weld operation to indicate when axial upset amount is withinand outside the predetermined upset range.
 2. In an inertial weldingprocess of the type wherein during each of a plurality of repetitivecycles, two workpieces are relatively rotated in rubbing contact at acommon interface solely by means of a rotating inertial mass while beingcontinuously pressed together, an improved method of maintaining qualitycontrol during repetitive cycles of said inertial welding processcomprising the steps of:Monitoring the amount of relative axial movementbetween the workpieces to determine axial upset amount during eachwelding cycle and generating an electrical signal proportional to theaxial upset amount between the workpieces; communicating said signalthrough a start time delay means effective to prevent transmission ofthe signal therethrough for a predetermined time period after initialengagement of workpieces; communicating an output signal from the starttime delay means; comprising said output signal from the start timedelay means with a signal predetermined as being representative of aneffective bond between the workpieces; producing a substantiallyinstantaneous signal indicating correlation between the axial upsetamount and the predetermined upset range; and conditioning repetition ofthe welding operation upon maintenance of said axial upset amount signalwithin the predetermined range representative of a effective bond; thepredetermined time period being selected in accordance with thecharacteristics of the workpieces and the parameters of the particularweld cycle in order to provide more accurate quality control in thefinished weld.
 3. A method as in claim 2 further comprising the step ofpermanently recording the signal proportional to axial upset amountbetween the workpieces.
 4. A quality control method as in claim 3,wherein said start time delay means is adjusted to prevent start of saidcomparing for approximately one second when the workpieces are steelparts having an interface area of approximately 3-12 square inches.